In recent years, surface mount technology with high density has been widely adopted. Such surface mount technologies are classified, among others, into double-sided surface mount technology in which chip type parts are joined with use of solder paste, and hybrid mount technology which is a combination of surface mount technology of chip type parts using solder paste and through-hole mount technology of discrete parts. In either mount process, a printed wiring board is subjected to two or more soldering steps, and thus it is exposed to high temperatures resulting in a severe thermal history.
Oxide film formation is accelerated by heating the surface of copper or copper alloys constituting the circuit part of a printed wiring board, and thus the surface of the circuit part cannot maintain good solderability.
In order to protect the copper circuit part of the printed wiring board from air oxidation, a chemical layer is generally formed on the surface of the circuit part using a surface treating agent. It is necessary, however, that good solderability be maintained by preventing the chemical layer from degenerating (i.e., being degraded) to protect the copper circuit part even after the copper circuit part has a thermal history of multiple cycles.
Tin-lead alloy eutectic solders have been conventionally used for mounting electronic components to a printed wiring board, etc. In recent years, however, concerns have developed that the lead contained in the solder alloy adversely affects the human body, and thus the use of lead-free solder is desired.
Accordingly, various lead-free solders are being considered. For example, lead-free solders have been suggested in which one or more metals, such as silver, zinc, bismuth, indium, antimony, copper, etc., are added to a base metal of tin.
The conventionally used tin-lead eutectic solder is excellent in wettability on the surface of substrate, particularly copper, and thus strongly adheres to copper, resulting in high reliability. In contrast, lead-free solder is inferior to the conventionally used tin-lead solder in wettability on a copper surface, and thus exhibits poor solderability and low bonding strength due to voids and other bonding defects.
Therefore, when using lead-free solder, it is necessary to select a solder alloy with superior solderability and a flux which is suitable for use with lead-free solder. A surface treatment agent for use in preventing oxidation on the surface of copper or a copper alloy is also required to have functions for improving the wettability and solderability of the lead-free solder.
Many lead-free solders have a high melting point, and a soldering temperature that is about 20 to about 50° C. higher than that of the conventionally used tin-lead eutectic solder. Thus, the surface treatment agent for use in the process of soldering with lead-free solder should have the characteristic of being able to form a chemical layer with excellent heat resistance.
Many imidazole compounds, such as 2-alkylimidazole, 2-arylimidazole, 2-alkylbenzimidazole, 2-arylbenzimidazole, 2-aralkylbenzimidazole compounds, etc., have been considered as active ingredients for a surface treatment agent. All of these imidazole compounds, however, have been considered for use with a tin-lead eutectic solder, and thus give insufficient wettability, resulting in inadequate solderability when used for use in the process of soldering with lead-free solder.
U.S. Pat. Nos. 5,498,301 and 5,560,785 and EP 0627499 A1, for example, propose a surface treatment agent containing as an active ingredient an imidazole compound having an aryl group at the 2- and 4-positions of an imidazole ring and represented by general formula (A),
wherein R is hydrogen or methyl; R1 and R2 are hydrogen, lower alkyl or halogen; and R3 and R4 are hydrogen, lower alkyl, halogen, lower alkoxy, di-lower alkylamino, cyano or nitro. These references disclose in Example 10 a surface treatment agent containing 2-(2,4-dichlorophenyl)-4-phenyl-5-methylimidazole as an active ingredient, and disclose that tests for solder wettability and spreadability of a solder paste were carried out using said surface treatment agent.
However, the solder used in these tests was a tin-lead eutectic solder, and the references are totally silent as to whether solder wettability is improved and good solderability is attained when lead-free solder is used.
Japanese Unexamined Patent Publication No. 7-243053 discloses 2,4-diphenylimidazole and 2,4-diphenyl-5-methylimidazole as a surface treatment agent for copper and copper alloys. It is also disclosed in Examples thereof that 2,4-diphenylimidazole and 2,4-diphenyl-5-methylimidazole are used as a surface treatment agent for copper and copper alloys, particularly for improving the solder wettability and solder flow-up rate of a solder paste containing lead.
This publication also discloses a process for synthesizing 2,4-diphenylimidazole as illustrated by the following reaction scheme (1):
wherein X is chlorine or bromine, and additionally a process for synthesizing 2,4-diphenyl-5-methylimidazole as illustrated by the following reaction scheme (2):

U.S. Pat. No. 5,498,301 (p. 4) and EP 0627499 A1 (p. 5) corresponding to Japanese Unexamined Patent Publication No. 7-243053 suggest that 2-phenyl-4-(dichlorophenyl)imidazole and 2-phenyl-4-(dichlorophenyl)-5-methylimidazole in which hydrogen atoms of phenyl at the 4-position are substituted with two chlorine atoms are useful as a surface treatment agent for copper, in particular as a treatment agent for improving the solder wettability and solder flow-up rate of a solder paste containing lead. Further, these references suggest a process for producing an imidazole compound having one or more substituents at each phenyl group of the 2- and 4-positions by the following reaction schemes (3) and (4).
wherein R1 and R2 are hydrogen, lower alkyl or halogen; and R3 and R4 are hydrogen, lower alkyl, halogen, lower alkoxy, di-lower alkylamino or nitro.
wherein R1a and R2a are hydrogen, lower alkyl or halogen; and R3a and R4a are hydrogen, lower alkyl, halogen, lower alkoxy, di-lower alkylamino or nitro.
Neither U.S. Pat. No. 5,498,301 nor EP 0627499 A1, however, disclose that 2-phenyl-4-(dichlorophenyl)imidazole and 2-phenyl-4-(dichlorophenyl)-5-methylimidazole were actually synthesized. No CAS Registry Number is given to these compounds either.
Especially, the reaction scheme (4) seemingly suggests that 2-phenyl-4-(dichlorophenyl)-5-methylimidazole compounds having a methyl group at the 5-position can be synthesized from benzaldehyde, 1-(dichlorophenyl)-1,2-propanedione compound and ammonium acetate. Actually, however, while benzaldehyde and ammonium acetate are readily available as industrial chemicals, said 1-(dichlorophenyl)-1,2-propanedione compound is not commercially available as an industrial chemical or as a reagent, and thus cannot be obtained unless synthesized,
A process for synthesizing 1-phenyl-1,2-propanedione compounds is disclosed in U.S. Pat. No. 4,107,210. The inventors attempted to synthesize 1-(2,4-dichlorophenyl)-1,2-propanedione and 1-(3,4-dichlorophenyl)-1,2-propanedione according to the disclosed process. However, the desired products could not be isolated since the reaction was complicated and an appropriate purification method could not be found in either case. Thus, 2-phenyl-4-(dichlorophenyl)-5-methylimidazole compounds have not yet been successfully synthesized based on the reaction scheme (4).
The above-described U.S. Pat. No. 5,498,301 (the left column, line 27, page 4, and Example 10 in the right column, page 7), U.S. Pat. No. 5,560,785 (left column, line 24, page 4, and Example 10 in the left column, page 7) and EP 0627499 A1 (first line, page 6, and Example 10 on page 10) disclose using 2-(2,4-dichlorophenyl)-4-phenyl-5-methylimidazole as a surface treatment agent for copper.
However, other imidazole compounds in which phenyl at the 2-position is substituted with two chlorine atoms are not known.